Abstract: The active sub-array is an important part of phased array radar. This article introduces the basic architecture and composition of the active sub-array. The key technologies of structure design such as overall system design, connection design, cooling design and application of microelectromechanical systems (MEMS) are discussed. The structure design of the active sub-array of a vehicle-borne phased array radar is presented as an example. It provides a reference for subsequent structure design of active sub-array.